In electrophotographic reproduction apparatus and printers, an electrostatic latent image is formed on a photoconductive imaging member by first uniformly charging the imaging member and then image-wise exposing the imaging member using various light sources such as a scanned laser, LED array, optical flash, or other suitable, known methods. The electrostatic latent image is then developed into a visible image by bringing the imaging member into close proximity with an electrostatic developer that includes charged marking particles. In a 2-component developer, marking particles are mixed with larger, magnetic particles called carrier particles, where the marking particles become triboelectrically charged by contact with the carrier particles. The developer is contained in a development station that typically includes a roller with a magnetic core. The carrier particles transport the marking particles into contact with the imaging member bearing the electrostatic latent image. The development station is suitably biased and the marking particles suitably charged so that the proper amount of marking particles, are deposited in electrostatic image the regions of the imaging member.
After the electrostatic latent image on the imaging member has been developed, the developed image is generally transferred to a receiver member, such as sheets of paper or transparency stock. This is generally accomplished by applying an electric field in such a manner to urge the marking particles from the imaging member to the receiver member. In some instances, it is preferable to first transfer the developed image from the imaging member to an intermediate member and then from the intermediate member to the receiver member. Again, this is most commonly accomplished by applying an electric field to urge the developed image toward the appropriate receiver member. The receiver member bearing the developed image is then passed through a fusing device to permanently affix the developed image to the receiver member by heat and/or pressure. The marking particles are typically a thermoplastic polymer that is electrically non-conductive. The process of transferring the developed image to the receiver results in a polar electrostatic charge on the surfaces of the marking particle image and the receiver member. The polar charge will dissipate through a conductive receiver member such as moisture-containing paper, but will not migrate through the insulating marking particle image layer or the insulating receiver member. The result is trapped electrostatic charge on the imaged receiver member. On a conducting receiver member, the amount of trapped charge is dependent on the coverage of the marking particle image. The trapped charge can inconvenience the user of the printed receiver members, as the member sheets will tend to stick together by the electrostatic forces of attraction between the charges on adjacent sheets.
It is well known to use an ac corona discharge to dissipate electrostatic charges on the surfaces of moving webs. For example, two AC corona chargers, facing each other on opposite sides of the web are typically used. Current regulated, high voltage AC power supplies are typically used to power the corona chargers in these exemplary configurations. In attempting to use a pair of corona chargers in this way to discharge receiver members, new problems are encountered due to the interframe spaces between successive receiver members. The resistivity between the two opposing chargers changes significantly when a receiver member is between the two chargers, versus during an interframe when no receiver member is between the two chargers. With pure current regulation the corona wire voltage can increase to critical, high values when a receiver member is in between the two chargers. The voltage would also vary with different types of receiver members because of the variation in receiver member resistivity. When a highly resistive member exits the area between the opposed chargers it is possible for an arc to develop between the opposing corona wires. This can happen before the current regulation control of the power supply can reduce the output voltage of the supply to react to the change in resistance between the corona wires.
Arcing results in undesired electrical noise radiated into the control system of the reproduction apparatus and possibly to the environment around the machine. Arcing can also be damaging to the apparatus hardware and materials. If, to address this problem, a pure peak to peak voltage regulating power supply is used, the current would reach critical, high levels when the interframe, between successive receiver member, is in between the two chargers. In this mode, the chargers will be operating at an unnecessarily high power level and generate excessive heat in the power supply. At the corona wire, the corona emission and the resultant chemical emissions will also be unnecessarily high.
In co-pending U.S. patent application Ser. No. 09/866,182 (in the name of Hasenauer), a novel power supply is disclosed, hereafter referred to as the Hasenauer power supply, for powering a pair of opposed AC corona chargers for dissipating the trapped polar charge on the receiver members, that solves these problems. The Hasenauer power supply uses a combination of both output control methods, current regulation, and peak to peak voltage regulation, to provide a solution that prevents arcing and over-current loading for receiver member sheet fed applications. Driven by the resistance between the two chargers, the power supply changes automatically from current regulation to voltage limit mode. However, due to limited output current capacity, the Hasenauer power supply becomes less effective at very high receiver member speeds.